Robot end effector position error correction using auto-teach methodology

ABSTRACT

A robot arm positioning error is corrected by employing a specimen gripping end effector in which a light source and a light receiver form a light transmission pathway that senses proximity to the specimen. A robot arm old position is sensed and recorded. The robot arm retrieves the specimen from the old position and employs old position information to replace the specimen at a new position that is ideally the same as the old position. A robot arm new position is sensed and recorded. A difference between the new and old positions represents a position error. A correct position is obtained by processing the position error and the old position information.

RELATED APPLICATIONS

Not Applicable

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

TECHNICAL FIELD

This invention relates to specimen handling robots, and moreparticularly, to an edge gripping semiconductor wafer robot arm endeffector positioning error reducing method.

BACKGROUND OF THE INVENTION

Integrated circuits are produced from wafers of semiconductor material.The wafers are typically housed in a cassette having a plurality ofclosely spaced slots, each of which can contain a wafer. The cassette istypically moved to a processing station where the wafers are removedfrom the cassette, placed in a predetermined orientation by a prealigneror otherwise processed, and returned to another location for furtherprocessing.

Various types of wafer handling devices are known for transporting thewafers to and from the cassette and among processing stations. Manyemploy a robotic arm having a spatula-shaped end that is inserted intothe cassette to remove or insert a wafer. The end of the robotic arm isreferred to as an end effector that typically employs a vacuum toreleasibly hold the wafer to the end effector. The end effectortypically enters the cassette through the narrow gap between a pair ofadjacent wafers and engages the backside of a wafer to retrieve it fromthe cassette. The end effector must be thin, rigid, and positionablewith high accuracy to fit between and not touch the closely spaced apartwafers in the cassette. After the wafer has been processed, the roboticarm inserts the wafer back into the cassette.

Unfortunately, positioning errors while transferring the wafer among thecassette, robot arm, and processing stations, such as a prealigner, maycause damage to the wafer and contamination of the other wafers in thecassette because intentional engagement as well as inadvertent touchingof the wafer may dislodge particles that can fall and settle onto theother wafers. Wafer damage can include scratches as well as metallic andorganic contamination of the wafer material. Reducing such positioningerrors and the resultant contamination is particularly important tomaintaining wafer processing yields.

What is needed, therefore, is a specimen gripping end effector that cansecurely, quickly, and accurately transfer semiconductor wafers whileminimizing positioning-related and other causes of wafer scratching andparticle contamination.

SUMMARY OF THE INVENTION

An object of this invention is, therefore, to provide a specimenhandling device that minimizes specimen damage and the production ofcontaminate particles.

Another object of this invention is to provide a semiconductor waferhandling device that can quickly and accurately transfer semiconductorwafers between a wafer cassette and a wafer processing station.

A further object of this invention is to provide a wafer handling devicepositioning error correction method.

Robot arm end effectors of this invention rapidly and cleanly transfersemiconductor wafers between a wafer cassette and a processing station.The end effectors include at least one proximal rest pad and at leasttwo distal rest pads having pad and backstop portions that support andgrip the wafer within an annular exclusion zone that extends inward fromthe peripheral edge of the wafer. The end effectors also include anactive contact point that is movable between a retracted wafer-loadingposition and an extended wafer-gripping position. The active contactpoint is movable to urge the wafer against the distal rest pads so thatthe wafer is gripped only at its edge or within the exclusion zone. Theend effectors are configured so that wafer edge contact is achieved forend effectors with inclined rest pads. Optical sensors detect retracted,safe specimen loading/gripping, and extended positions of the activecontact point.

The end effectors are generally spatula-shaped and have a proximal endthat is operably connected to a robot arm. The active contact point islocated at the proximal end, which allows the end effector to belighter, stronger, and more slender than end effectors having movingmechanisms that may not fit between adjacent wafers in a cassette. Thelack of moving mechanisms further causes the end effector to produceless contamination within the cassette. Additionally, locating theactive contact point at the proximal end of the end effector ensuresthat it is remote from harsh conditions such as heated environments andliquids.

A vacuum pressure-actuated piston moves the active contact point betweena retracted position, in which the wafer is loaded into the endeffector, and an extended position in which the wafer is gripped. Afirst embodiment of the piston employs vacuum pressure to move theactive contact point between extreme positions; a second embodiment ofthe piston employs vacuum pressure to retract the active contact pointand a spring to extend the active contact point; and a third embodimentof the piston adds the above-mentioned optical sensors for detectingretracted, safe specimen loading/gripping, and extended positions of theactive contact point.

Alternative embodiments of the end effector include flat or inclined,narrow or arcuate rest pads onto which the wafer is initially loaded.The narrow and arcuate inclined rest pad embodiments assist in centeringand gripping the wafer between the active contact point and the distalrest pads. The arcuate rest pads more readily accommodate gripping andhandling flatted wafers.

The end effectors further include fiber optic light transmission sensorsfor accurately locating the wafer edge and bottom surface. The sensorsprovide robot arm extension, elevation, and positioning data thatsupport methods of rapidly and accurately placing a wafer on andretrieving a wafer from a wafer transport stage or a process chamber,and placing a wafer in and retrieving a wafer from among a stack ofclosely spaced wafers stored in a wafer cassette. The methods includeposition error sensing and correction for effectively preventingaccidental contact between the end effector and adjacent wafers stackedin a cassette.

Additional aspects and advantages of this invention will be apparentfrom the following detailed description of preferred embodiments, whichproceed with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a first embodiment of the end effector of thisinvention shown inserted into a semiconductor wafer cassette to retrieveor replace a wafer.

FIG. 2 is a side elevation view of the end effector of FIG. 1 withoutthe wafer cassette but showing the end effector inserted between anadjacent pair of three closely spaced apart wafers as they would bestored in the cassette.

FIG. 3 is an enlarged side elevation view of a flat rest pad embodimentof this invention showing the rest pad engaging an exclusion zone of awafer.

FIG. 4 is an enlarged side elevation view of an inclined rest padembodiment of this invention showing the inclined rest pad engagingsubstantially a periphery of a wafer.

FIG. 5 is a plan view of a second embodiment of the end effector of thisinvention shown gripping a semiconductor wafer and adjacent to asemiconductor wafer in a wafer cassette to sense, retrieve, or replace awafer.

FIG. 6 is a sectional side elevation view of the end effector of FIG. 5showing an active contact point actuating mechanism gripping a waferbetween adjacent ones of closely spaced apart wafers as they would bestored in the wafer cassette.

FIG. 7 is an enlarged isometric view of a distal arcuate rest padembodiment of this invention mounted on the distal end of the endeffector of FIG. 5.

FIG. 8 is an end perspective view of the end effector of FIG. 5 showingpositional relationships among the movable contact point, arcuate restpads, and wafer edge and elevation sensors of the second embodiment endeffector of this invention.

FIG. 9 is a bottom view of the end effector of FIG. 5 showing fiberoptic routing channels for elevation sensors of the second embodimentend effector of this invention.

FIG. 10 is a fragmentary plan view of a portion of a third embodiment ofan end effector of this invention, showing positional relationshipsamong the wafer, a position sensing active contact point actuatingmechanism, and the proximal rest pads.

FIG. 11 is a sectional side elevation view of the end effector portionof FIG. 10 showing the position sensing active contact point actuatingmechanism fully extended between adjacent closely spaced wafers as theywould be stored in the wafer cassette.

FIG. 12 is an overall plan view of the end effector of FIG. 10 showingalternate wafer gripping and sensing positions.

FIGS. 13A and 13B are plan views of a robot arm and an end effector ofthis invention shown in three angularly displaced positions for sensingradial distances to an edge of a wafer (shown reduced in size forclarity).

FIGS. 14A and 14B are respective side elevation and plan views of anexemplary two-arm, multiple link robot arm system from which the endeffector of the present invention extends.

FIG. 15 is a side elevation view in stick diagram form showing the linkcomponents and the associated mechanical linkage of the robot arm systemof FIGS. 14A and 14B.

FIG. 16 is an isometric view in stick diagram form showing therotational motion imparted by the motor drive links of the mechanicallinkage of the robot arm system of FIGS. 14A and 14B.

FIG. 17A is a diagram showing the spatial relationships and parametersthat are used to derive control signals provided by, and FIG. 17B is ablock diagram of, the motor controller for the robot arm system of FIGS.14A and 14B.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 and 2 show a first embodiment of a spatula-shaped end effector10 of this invention for transferring semiconductor wafers, such as awafer 12 (shown transparent to reveal underlying structures), to andfrom a wafer cassette 14. End effector 10 is adapted to receive andsecurely hold wafer 12 and transfer it to and from cassette 14 forprocessing. FIG. 2 shows that end effector 10 is particularly adaptedfor retrieving and replacing wafer 12 from among closely spaced wafers,such as wafers 12, 12A, and 12B, which are shown as they might bestacked in wafer cassette 14. Wafers having diameters of less than 150mm are typically spaced apart at a 4.76 mm ({fraction (3/16)} inch)pitch distance; 200 mm diameter wafers are typically spaced apart at a6.35 mm (¼ inch) pitch distance; and 300 mm wafers are typically spacedapart at a 10 mm (0.394 inch) pitch distance.

End effector 10 is operably attached to a robot arm 16 (a portion ofwhich is shown) that is programmably positionable in a well knownmanner. In general, end effector 10 enters wafer cassette 14 to retrievewafer 12 positioned between wafers 12A and 12B. End effector 10 is thenfinely positioned by robot arm 16 and actuated to grip a periphery 18 ofwafer 12, remove wafer 12 from cassette 14, and transfer wafer 12 to aprocessing station (not shown) for processing. End effector 10 may then,if necessary, reinsert wafer 12 into cassette 14, release wafer 12, andwithdraw from cassette 14.

End effector 10 is operably coupled to robot arm 16 at a proximal end 20and extends to a distal end 22. End effector 10 receives wafer 12between proximal end 20 and distal end 22 and includes at least two and,preferably, four rest pads upon which wafer 12 is initially loaded. Twodistal rest pads 24 are located at, or adjacent to, distal end 22 of endeffector 10; and at least one, but preferably two proximal rest pads 26are located toward proximal end 20. Distal rest pads 24 mayalternatively be formed as a single arcuate rest pad having an angularextent greater than the length of a “flat,” which is a crystalstructure-indicating feature commonly found on semiconductor wafers. Aflat 27 is shown, by way of example only, positioned between proximalrest pads 26. Of course, wafer 12 may have a different orientation, soperiphery 18 is also shown positioned between proximal rest pads 26.

Wafer 12 includes an exclusion zone 30 (a portion of which is shown indashed lines). Semiconductor wafers have an annular exclusion zone, orinactive portion, that extends inwardly about 1 mm to about 5 mm fromperiphery 18 and completely surrounding wafer 12. Exclusion zone 30 isdescribed as part of an industry standard wafer edge profile template inSEMI specification M10298, pages 18 and 19. As a general rule, no partof end effector 10 may contact wafer 12 beyond the inner boundary ofexclusion zone 30. It is anticipated that future versions of thespecification may allow edge contact only, a requirement that is readilyaccommodated by this invention.

The distance between rest pads 24 and the distance between rest pads 26each have an angular extent greater than any feature on wafer 12 toguarantee that wafer 12 is gripped only within exclusion zone 30. Restpads 24 and 26 may be made of various materials, but a preferredmaterial is polyetheretherketone (“peek”), which is a semi-crystallinehigh temperature thermoplastic manufactured by Victrex in the UnitedKingdom. The rest pad material may be changed to adapt to differentworking environments, such as in high temperature applications.

FIG. 3 shows a substantially flat embodiment of distal rest pads 24.This embodiment can be advantageously, but need not exclusively be, usedwith wafers having less than about a 200 mm diameter. Distal rest pads24 include a pad portion 32 and a backstop portion 34. In the flatembodiment, pad portion 32 is substantially parallel to an imaginaryplane 36 extending through wafer 12, and backstop portion 34 is inclinedtoward wafer 12 at a backstop angle 38 of up to about 5 degrees relativeto a line perpendicular to plane 36. Alternatively, pad portion 32 maybe inclined away from wafer 12 up to about 3 degrees relative to plane36. Pad portion 32 has a length 40 that is a function of the depth ofexclusion zone 30, but is preferably about 3 mm long. Wafer 12 typicallyhas a substantially rounded peripheral edge and contacts rest pads 24only within exclusion zone 30. Wafer 12 is gripped by urging it into theincluded angle formed between pad portion 32 and backstop portion 34.

FIG. 4 shows an inclined embodiment of distal rest pads 24. Thisembodiment can be advantageously, but need not exclusively be, used withwafers having greater than about a 200 mm diameter. Distal rest pads 24include an inclined pad portion 42 and a backstop portion 34. In theinclined embodiment, inclined pad portion 42 is inclined away from wafer12 at a rest pad angle 44 of about 3 degrees relative to plane 36, andbackstop portion 36 is inclined toward wafer 12 at backstop angle 38 ofup to about 3 degrees. Inclined pad portion 42 has a length 40 that is afunction of the depth of exclusion zone 30, but is preferably about 3 mmlong. As before, wafer 12 typically has a substantially roundedperipheral edge and contacts rest pads 24 only within exclusion zone 30.Wafer 12 is gripped by urging it into the included angle formed betweenpad portion 42 and backstop portion 34. In the inclined embodiment,there is substantially no contact between rest pad 24 and a bottomsurface 46 of wafer 12. This rest pad embodiment is also suitable forwafer edge contact only.

Both the flat and inclined embodiments of distal rest pads 24 have aheight 48 that substantially reaches but does not extend beyond the topsurface of wafer 12.

Referring again to FIG. 1, proximal rest pads 26 are similar to distalrest pads 24 except that each rest pad 26 does not necessarily require abackstop portion and its pad portion has a length of about twice that oflength 40.

End effector 10 further includes an active contact point 50 that islocated at proximal end 20 of end effector 10 and between proximal restpads 26. Active contact point 50 is movable between a retractedwafer-loading position (shown in dashed lines) and an extendedwafer-gripping position (shown in solid lines).

Active contact point 50 is operatively connected to a piston 52 forreciprocation between the retracted and extended positions. In a firstembodiment, piston 52 reciprocates within a bore 54 and is preferablyvacuum pressure operated to extend and retract active contact point 50.Active contact point 50 is connected to piston 52 by a piston rod 56that extends through an airtight seal 58. Bore 54 forms a vacuum chamberin end effector 10 that is divided by piston 52 into a drive chamber 60and a return chamber 62. Drive chamber 60 is in pneumatic communicationwith a vacuum pressure source (not shown) through a first channel 64,and return chamber 62 is in pneumatic communication with the vacuumpressure source through a second channel 66. The vacuum pressure actsthrough drive chamber 60 against the front face of piston 52 to extendactive contact point 50 to the wafer-gripping position and acts throughreturn chamber 62 against the back face of piston 52 to retract activecontact point 50 as controlled by the programmable control. The vacuumpressure source is routed to first and second channels 64 and 66 throughrotary vacuum communication spools in robot arm 16. Preferred rotaryvacuum communication spools are described in U.S. Pat. No. 5,741,113 forCONTINUOUSLY ROTATABLE MULTIPLE LINK ROBOT ARM MECHANISM, which isassigned to the assignee of this application.

Piston 52 further includes an annular groove 68 that is in pneumaticcommunication with a vent (not shown) in piston rod 56. First and secondchannels 64 and 66 are connected to, respectively, drive chamber 60 andreturn chamber 62 at locations that are opened to groove 68 at thetravel limits of piston 52. Therefore, vacuum pressure in first andsecond channels 64 and 66 is reduced at the travel limits of piston 52,thereby providing signals to the vacuum controller that active contactpoint 50 is fully extended or retracted to effect proper loading ofwafer 12.

After wafer 12 is loaded onto end effector 10, active contact point 50is actuated to move wafer 12 into its gripped position. As activecontact point 50 is extended, it urges wafer 12 toward distal rest pads24 until wafer 12 is gripped within exclusion zone 30 by active contactpoint 50 and distal rest pads 24.

Proximal rest pads 26 are arranged relative to distal rest pads 24 sothat plane 36 of wafer 12 is preferably parallel to end effector 10 whengripped. This arrangement is readily achieved when the flat embodimentof proximal and distal rest pads 24 and 26 is employed. However, whenthe inclined embodiment is employed, proximal and distal rest pads 24and 26 are arranged such that the points where wafer 12 contacts padportions 42 are substantially equidistant from a center 70 of wafer 12when active contact point 50 is extended and wafer 12 is gripped. Forexample, when wafer 12 is in the position shown in FIG. 1, the padportions of distal and proximal rest pads 24 and 26 contact wafer 12 atpoints tangent to periphery 18 such that a line through the center ofeach pad portion 42 intersects center 70 of wafer 12.

The location of active contact point 50 at proximal end 20 allows endeffector 10 to be lighter, stronger, and more slender than end effectorshaving moving mechanisms that may not fit between adjacent wafers 12,12A, and 12B in cassette 14. The lack of moving mechanisms furthercauses end effector 10 to produce less contamination within cassette 14.Additionally, locating active contact point 50 at proximal end 20 of endeffector 10 ensures that active contact point 50 is remote from harshconditions such as heated environments and liquids.

The close spacing of adjacent wafers 12, 12A, and 12B requires accuratepositioning of end effector 10 to enter cassette 14 and positively gripthe wafers without touching adjacent wafers.

FIGS. 5 and 6 show a second embodiment of a spatula-shaped end effector110 of this invention for transferring semiconductor wafers, such aswafer 12 (shown transparent to reveal underlying structures), to andfrom wafer cassette 14 (not shown in this view). End effector 110 issimilar to end effector 10 but is further adapted to sense the bottomsurface of a wafer stored in wafer cassette 14 without protruding intothe cassette. FIG. 6 shows that end effector 110 is particularly adaptedfor retrieving and replacing wafer 12 from among closely spaced apartwafers, such as wafers 12, 12A, and 12B, which are shown as they mightbe stacked in wafer cassette 14.

End effector 110 is operably attached to robot arm 16. In general, endeffector 110 senses the bottom surface of wafer 12 before entering wafercassette 14 to retrieve wafer 12 from between wafers 12 A and 12B. Endeffector 110 is then finely positioned by robot arm 16 and actuated togrip periphery 18 of wafer 12, remove wafer 12 from cassette 14, andtransfer wafer 12 to a processing station (not shown) for processing.End effector 110 may then, if necessary, reinsert wafer 12 into cassette14, release wafer 12, and withdraw from cassette 14.

End effector 110 is operably coupled to robot arm 16 at a proximal end120 and extends to a distal end 122. End effector 110 receives wafer 12between proximal end 120 and distal end 122 and preferably includes atleast two and, more preferably, four arcuate rest pads upon which wafer12 is initially loaded. Two distal arcuate rest pads 124 are located at,or adjacent to, distal end 122 of end effector 110; and at least one,but preferably two proximal arcuate rest pads 126 are located towardproximal end 120. Distal and proximal arcuate rest pads 124 and 126 mayhave an angular extent greater than flat 27, which is shown, by way ofexample only, positioned between proximal rest pads 126. Of course,wafer 12 may have a different orientation from that shown.

Arcuate rest pads 124 and 126, whether separated as shown, or joinedinto a single rest pad, have an angular extent greater than any featureon wafer 12 to guarantee that wafer 12 is sufficiently gripped, whetherflatted or not, and only within exclusion zone 30. Like rest pads 24 and26, rest pads 124 and 126 may be made of various materials, but thepreferred material is peek.

FIG. 7 shows the embodiment of distal arcuate rest pads 124 that issuitable for use with flatted or unflatted wafers. Distal arcuate restpads 124 include an inclined pad portion 132 and a backstop portion 134.Referring also to FIG. 4, inclined pad portion 132 is inclined away fromwafer 12 at rest pad angle 44 of about 3 degrees relative to plane 36,and backstop portion 136 is inclined toward wafer 12 at backstop angle38 of up to about 3 degrees. Inclined pad portion 132 has a length 140that is a function of the depth of exclusion zone 30, but is preferablyabout 3 mm long. As before, wafer 12 typically has a substantiallyrounded peripheral edge and contacts arcuate rest pads 124 by wafer edgecontact (and perforce only within exclusion zone 30). Of course, theperipheral edge need not be rounded. Wafer 12 is gripped by urging itinto the included angle formed between inclined pad portion 132 andbackstop portion 134.

Distal arcuate rest pads 124 have a height 148 that substantiallyreaches but does not extend beyond the top surface of wafer 12.

Referring again to FIG. 5, proximal arcuate rest pads 126 are similar todistal arcuate rest pads 124 except that each rest pad 126 does notnecessarily require a backstop portion and its pad portion has a lengthof about twice that of length 140.

End effector 110 further includes an active contact point 150 that islocated at proximal end 120 of end effector 110 and between proximalarcuate rest pads 126. Active contact point 150 is movable between aretracted wafer-loading position (not shown) and the extendedwafer-gripping position shown.

Referring again to FIG. 6, a second embodiment of an active contactpoint actuating mechanism 151 is shown employed with end effector 110.Active contact point 150 is operatively connected to a piston 152 forreciprocation between retracted and extended positions. In thisembodiment, piston 152 reciprocates within a bore 154 and is urged by aspring 155 to extend active contact point 150 and by a vacuum pressureto retract active contact point 150. Active contact point 150 isconnected to piston 152 by a piston rod 156 that extends through anannular airtight seal 158. Bore 154 includes an end cap 159 that formsone wall of a vacuum chamber 160, the other wall of which is movablyformed by piston 152. Vacuum chamber 160 is in pneumatic communicationwith a vacuum pressure source (not shown) through a vacuum feedthrough162 and a vacuum channel 164. Spring 155 presses against the face ofpiston 152 to extend active contact point 150 to the wafer-grippingposition, whereas the vacuum pressure acts through vacuum chamber 160against the face of piston 152 to overcome the spring force and retractactive contact point 150 to the wafer-releasing position.

In the second embodiment, active contact point 150 is urged againstwafer 12 with a force determined solely by spring 155. Spring 155 issupported between recesses 166 in piston 152 and end cap 159. The vacuumpressure source is routed to vacuum channel 164 through rotary vacuumcommunication seals or spools in robot arm 16.

Actuating mechanism 151 further includes a vent 168 in pneumaticcommunication with the atmosphere to allow free movement of piston 152within the portion of bore 154 not in pneumatic communication with thevacuum pressure source. Actuating mechanism 151 is made “vacuum tight”by O-ring seals 170 surrounding end cap 159 and vacuum feedthrough 162and by an annular moving seal 172 surrounding piston 152. O-ring bumperseals 174 fitted to the faces of piston 152 absorb contact shockspotentially encountered by piston 152 at the extreme ends of its travel.

After wafer 12 is loaded onto end effector 110, active contact point 150is actuated to move wafer 12 into its gripped position. As activecontact point 150 is extended by spring 155, it urges wafer 12 towarddistal arcuate rest pads 124 until wafer 12 is gripped by wafer edgecontact (and perforce within exclusion zone 30) by active contact point150 and distal arcuate rest pads 124. Active contact point 150 includesan inwardly inclined face portion 176 that urges wafer 12 towardproximal arcuate rest pads 126, thereby firmly gripping the peripheraledge of wafer 12.

Proximal arcuate rest pads 126 are arranged relative to distal arcuaterest pads 124 so that the plane of wafer 12 is preferably parallel toend effector 110 when gripped.

In a manner similar to end effector 10, the location of active contactpoint 150 at proximal end 120 allows end effector 110 to be lighter,stronger, and more slender than end effectors having moving mechanismsthat may not fit between adjacent wafers 12, 12A, and 12B in cassette14. The lack of moving mechanisms between its proximal and distal endsfurther causes end effector 110 to produce less contamination withincassette 14. Moreover, unlike end effector 10, which is actuated by twovacuum lines, end effector 100 requires only one vacuum line foractuation. Of course, end effector 10 could be fitted with actuatingmechanism 151.

The close spacing of adjacent wafers 12, 12A, and 12B requires accuratepositioning of end effector 110 to enter cassette 14 and positively gripthe wafers without touching adjacent wafers.

FIGS. 5, 8, and 9 show respective top, end, and bottom views of anembodiment including elevation sensors that provide accurate wafer 12positioning data relative to end effector 110. The elevation sensor ishoused in distal end 122 of end effector 110. The elevation sensor isimplemented as follows. A light source fiber 196 is routed through afirst channel 184 formed in the bottom surface of end effector 110 andrunning between proximal end 120 and a first distal tine 188 proximal todistal end 122 of end effector 110. In like manner, a light receiverfiber 198 is routed through a second channel 186 formed in the bottomsurface of end effector 110 and running between proximal end 120 and asecond distal tine 190 proximal to distal end 122 of end effector 110.Distal tines 188 and 190 are widely spaced apart across a gap 191 thatforms a relief region for certain types of processing equipment, such aswafer prealigners.

Fibers 196 and 198 terminate in mutually facing light path openings 192and 194 formed in distal tines 188 and 190. Fibers 196 and 198 formbetween them a wide opening that sights along a bottom surface chord 200of, for example, wafer 12A. Mutually facing light path openings 192 and194 form a narrow light transmission pathway 202 for detecting thepresence or absence of bottom surface chord 200 of wafer 12A. In endeffector 110, light transmission pathway 202 extends beyond the portionof distal end 122 that would first contact wafer 12, thereby furtherproviding an obstruction sensing capability. A light source/receivermodule 204 (FIG. 1) conventionally detects degrees of light transmissionbetween fibers 196 and 198 and, thereby, accurately senses thepositioning of bottom surface chord 200 between light path openings 192and 194. Of course, the relative positions of fibers 196 and 198 may bereversed.

The procedure by which end effector 110 accesses a predetermined waferfrom among closely spaced apart wafers in a cassette, is described belowwith reference to FIGS. 5, 6, and 8.

Active contact point 150 is placed in its retracted position.

End effector 110 is moved in an X direction toward cassette 14 untiltines 188 and 190 are adjacent to, but not touching, a predictedposition for any wafer 12 in cassette 14.

End effector 110 is then scanned in a Z direction such that lighttransmission pathway 202 intersects the bottom surface chord 200 of anywafer in cassette 14 and, additionally, detects any obstructionprojecting from cassette 14 toward end effector 110.

The controller (not shown) records the Z elevations of the bottomsurfaces of any wafers and obstructions detected.

Robot arm 16 is moved to a Z elevation calculated to access apredetermined wafer, such as wafer 12A, while also providing clearancefor end effector 110 between adjacent wafers.

The following optional operations may be performed:

End effector 110 may be optionally moved in an X direction towardcassette 14 until tines 188 and 190 are adjacent to, but not touching,wafer 12A. In this position, light transmission pathway 202 should beadjacent to bottom surface chord 200 of wafer 12A;

-   -   robot arm 16 is optionally moved in a Z direction until bottom        surface chord 200 of wafer 12A is sensed;    -   the controller optionally verifies the previously sensed Z        elevation of the bottom surface of wafer 12A; and    -   robot arm 16 is optionally moved in a −Z direction to provide        clearance for end effector 110 between adjacent wafers.

The controller moves end effector 10 in the Z direction calculated tocontact wafer 12A on landing pads 124 and 126.

Active contact point 150 is actuated to urge wafer 12A into the includedangle between pad and backstop portions 132 and 134 of distal arcuaterest pads 124, thereby gripping wafer 12A. (In FIG. 5, the gripped waferis shown as wafer 12).

End effector 110 withdraws wafer 12A in the −X direction from cassette14.

End effector 110 combines a very thin Z-direction profile and accuratewafer position sensing to enable clean, rapid, and secure movement ofvery closely spaced apart wafers in a cassette.

FIGS. 10, 11, and 12 show a third embodiment of a preferred fork-shapedend effector 210 of this invention for transferring semiconductorwafers, such as wafer 12 (shown transparent to reveal underlyingstructures), to and from wafer cassette 14 (not shown in these views).End effector 210 is similar to end effectors 10 and 110 but includes aposition sensing active contact point actuating mechanism 212 and distalend sensors 214 to accomplish various wafer sensing measurements.

FIG. 11 shows that end effector 210 is particularly suited forretrieving and replacing wafer 12 from among closely spaced apartwafers, such as wafers 12, 12A, and 12B, which are shown as they mightbe stacked in wafer cassette 14.

FIG. 12 shows end effector 210 operably coupled to robot arm 16 at aproximal end 216 and extending to forked distal ends 218 and 220. Endeffector 210 receives wafer 12 between proximal end 216 and forkeddistal ends 218 and 220 and preferably includes at least two and, morepreferably, four arcuate rest pads upon which wafer 12 is initiallyloaded. A distal arcuate rest pad 124 is located at, or adjacent to,each of forked distal ends 218 and 220; and at least one, but preferablytwo proximal arcuate rest pads 126 are located toward proximal end 216.End effector 210 also includes an active contact point 222 that islocated at proximal end 216 of end effector 210 and between proximalarcuate rest pads 126.

Referring to FIGS. 10 and 11, position sensing active contact pointactuating mechanism 212 is a third embodiment of the active contactpoint actuating mechanism. As in the second embodiment, active contactpoint 222 is operatively connected to piston 152 for reciprocationbetween fully retracted, fully extended, and intermediate positions.Piston 152 moves within bore 154 and is urged by a spring (FIG. 6) toextend active contact point 222 and by a vacuum pressure to retractactive contact point 222. Active contact point 222 is connected topiston 152 by piston rod 156 that extends through annular airtight seal158. Bore 154 includes end cap 159 that forms one wall of vacuum chamber160, the other wall of which is movably formed by piston 152. Vacuumchamber 160 is in pneumatic communication with the vacuum pressuresource (not shown) through vacuum feedthrough 162 and vacuum channel164. The spring presses against the face of piston 152 to extend activecontact point 222 to wafer-gripping and fully extended positions,whereas the vacuum pressure acts through vacuum chamber 160 against theface of piston 152 to overcome the spring force and retract activecontact point 222 to wafer-releasing and fully retracted positions.

Actuating mechanism 212 further includes vent 168 in pneumaticcommunication with the atmosphere to allow free movement of piston 152within the portion of bore 154 not in pneumatic communication with thevacuum pressure source. Actuating mechanism 212 is made “vacuum tight”by O-ring seals 170 surrounding end cap 159 and vacuum feedthrough 162,and by an annular moving seal 172 surrounding piston 152.

Unlike the first and second embodiments, actuating mechanism 212 furtherincludes a position indicating shaft 224 attached to piston 152 andextending axially through an annular seal 226 in end cap 159. A pair ofoptical interrupter switches 228 and 230 are mounted to a circuit board232 positioned just behind end cap 159 such that, depending on theposition of indicating shaft 224, it interrupts a pair of light beams234 and 236 in respective optical interrupter switches 228 and 230.

Optical interrupter switches 228 and 230 sense positions of activecontact point 222 corresponding to a retracted position region, a safegripping operation region, and an extended position region. (FIGS. 10and 11 show active contact point 222 in a fully extended position.)

The retracted position region ensures that wafer 12 is not gripped andis sensed when position indicating shaft 224 interrupts both of lightbeams 234 and 236.

The safe gripping operation region is a range of active contact point222 positions within which wafer loading, gripping, or unloadingoperation can be safely carried out and is sensed when positionindicating shaft 224 interrupts light beam 236 but not light beam 234.Moreover, when active contact point 222 is extended and comes to rest inthe safe gripping operation region, proper wafer gripping is verified.

The extended position region is a range of active contact point 222positions within which wafer 12 is not gripped and is sensed whenposition indicating shaft 224 interrupts neither of light beams 234 and236.

Optical interrupter switches 228 and 230 are in electrical communicationwith the above-referenced controller. The controller coacts with thevacuum pressure source actuating piston 152 to pulse or pressureregulate the amount of vacuum pressure and, thereby, control thepositions of active contact point 222. Of course, various other forms ofcontrollable motive forces may be employed to position active contactpoint 222.

In an operational example, active contact point 222 is moved to the safegripping operation region and a wafer 12 is loaded into end effector210. After wafer 12 is loaded, active contact point 222 is actuated tomove wafer 12 into its gripped position. As active contact point 150 isextended, it urges wafer 12 up inclined pad portions 132 of distalarcuate rest pads 124 until wafer 12 is gripped. Active contact point222 must be sensed in the safe gripping operating region to ensure thatwafer 12 is properly gripped.

Wafer 12 is released by retracting active contact point 222 to theretracted position region as sensed by position indicating shaft 224interrupting both of light beams 234 and 236. When wafer 12 is released,it slips back on inclined pad portions 132 of distal arcuate rest pads124, thereby providing sufficient clearance between wafer 12 andbackstop portion 134 for a safe Z-axis elevation move and retrieval ofend effector 210.

Referring again to FIG. 12, distal end sensors 214 of end effector 210are housed in forked distal ends 218 and 220. Distal end sensors 214 areimplemented as follows. A light source fiber is routed through a firstchannel 238 (shown in phantom lines) formed in the bottom surface of endeffector 210 and running between proximal end 216 and forked distal end218. In like manner, a light receiver fiber is routed through a secondchannel 240 (shown in phantom lines) formed in the bottom surface of endeffector 210 and running between proximal end 216 and forked distal end220. Forked distal ends 218 and 220 are widely spaced apart across a gap242 that forms a relief region for certain types of processingequipment, such as wafer prealigners.

The light fibers terminate in mutually facing light path openings (notshown) formed in forked distal ends 218 and 220. The fibers form betweenthem a wide opening that sights along the peripheral edge or the bottomsurface chord of a wafer. The mutually facing light path openings form anarrow light transmission pathway 244 for detecting the presence orabsence of the periphery or bottom surface chord of a wafer. Lighttransmission pathway 244 extends beyond the portion of forked distalends 218 and 220 that would first contact a wafer, thereby furtherproviding an obstruction sensing capability. As before, lightsource/receiver module 204 conventionally detects degrees of lighttransmission between the fibers and, thereby, senses any objects thatinterrupt light transmission pathway 244.

End effector 210 employs distal end sensors 214 to carry out waferperipheral edge and/or top and bottom chord sensing to perform variouswafer-related operations including: determining wafer absence orpresence in a cassette, Z-axis location in the cassette, protrusion fromthe cassette, tilt angle in the cassette, determining wafer rotationalcenter, thickness, center-to-center distance between the wafer and arobot arm rotational axis, and verifying end effector centroid location.Wafer sensing operations are described below with reference to lighttransmission pathway 244 of end effector 210, but they can also beaccomplished with light transmission pathway 202 of end effector 110.

FIG. 12 shows three alternative wafer positions. Wafer 12 (shown inphantom) is shown gripped by end effector 210, wafer 12A (shown in solidlines) is shown in a wafer edge sensing position, and wafer 12B (shownin phantom) is shown in a wafer chord sensing position.

Sensing wafer 12B protrusion from a cassette (not shown) entailsstepping (scanning) robot arm 16 up and down in the Z-axis direction andmoving end effector 210 in the X-axis direction at the start of eachscan until a protruding wafer, such as wafer 12B is detected. Priorrobot arm systems typically employed a dedicated protrusion sensor. Anyprotruding wafers are moved back into their proper positions, such asthe position shown by wafer 12A. When all wafers are in their properpositions, a final scan determines the Z-axis wafer locations in thecassette. Robot arm 16 X- and Z-axis movements are preferably in a fineresolution mode.

After light transmission pathway 244 is interrupted, indicating detectedpresence of wafer 12B, end effector 210 can locate wafer 12B top andbottom surfaces by moving end effector 210 downward in the Z-axisdirection until a top surface chord of wafer 12B interrupts lighttransmission pathway 244. End effector 210 continues moving downwarduntil light transmission pathway 244 is restored. This point representssensing a bottom surface chord of wafer 12B. End effector 210 is thenmoved to a Z-axis position midway between the points of interruption andrestoration of light transmission pathway. This Z-axis positionrepresents the approximate midpoint of wafer 12B thickness. Whilemaintaining this Z-axis position, end effector 210 is retracted in theX-axis direction until light transmission pathway 244 is restored,indicating that periphery 18 of the wafer has been detected. Wafer 12Ais shown in this position.

When end effector 210 is at the edge detection point represented bywafer 12A and because the radius of wafer 12A is known, the controllerand position encoders associated with robot arm 16 can determine theX-axis or radial distance to a center 246 of wafer 12A and a downwardZ-axis distance required to provide clearance between the bottom surfaceof wafer 12A and end effector 210. Knowing the clearance is necessarywhen placing and retrieving wafers from the cassette because the wafersare not necessarily parallel to end effector 210 and distances betweenadjacent wafers in the cassette can be tight.

End effector 210 further includes a controllable supination angle 248,which is the tilt angle about the X-axis of end effector 210 relative toa Y-axis. Wafers stacked horizontally in a cassette ordinarily havetheir major surface planes at a zero-degree tilt angle that should bematched by supination angle 248 of end effector 210. However, to reducewafer surface contamination, wafers are often stored vertically in acassette, which because the cassette storage slots are wider than thewafer thickness, allows the wafers assume an indefinite tilt angle. Tiltangle can be indefinite even after a cassette of wafers is returned to ahorizontal orientation. Therefore, the following procedure is followedto determine whether supination angle 248 equals the wafer tilt angle.In this procedure the wafers are in a horizontal orientation by way ofexample only.

Supination angle 248 of end effector 210 is set to about zero degrees.

Robot arm 16 moves end effector 210 in the X-axis direction such thatlight transmission pathway 244 intersects a chord of wafer 12B.

Robot arm 16 moves end effector 210 up and down in the Z-axis directionwhile dithering its supination angle 248 until a minimum thickness ofwafer 12B is computed.

The minimum thickness indicates that end effector 210 and the wafer arein the same datum plane and, therefore, supination angle 248substantially equals the tilt angle of wafer 12B.

Robot arm systems can be equipped with two end effectors or multiplearms. The technique described above for a controllable supination anglecan be extended to such multiple end effector systems by using a singlewafer as a reference to determine the X, Y, and Z dimension offsetsamong them.

Referring to FIGS. 13A and 13B, light transmission pathway 244 is alsoemployed to align robot arm 16 and end effector 210 with the somewhatindefinite position of wafer 12 in a cassette or on a prealigner(neither shown). Alignment is achieved, as shown in FIG. 13B, whencenter 252 of wafer 12, active contact point 222 (shown as a target) onend effector 210, and a shoulder axis 260 of robot arm 16 are allcollinear. Determining alignment preferably entails employing acylindrical coordinate system in which an r-axis extends radially fromshoulder axis 260, a Θ-axis extends angularly about shoulder axis 260,and a Z-axis extends coaxially with shoulder axis 260.

Performing alignment operations entails finding a minimum radialdistance r_(MIN) between shoulder axis 260 of robot arm 16 and theclosest point of periphery 18 on wafer 12 (wafer 12 is shown reduced insize to clarify this description). This is the same as determining amaximum extension distance r_(EXT) between shoulder axis 260 and lighttransmission pathway 244 for tangentially sensing periphery 18.Extension distances r_(EXT) are readable and controllable by the systemcontrolling robot arm 16.

FIG. 13A shows robot arm 16 and end effector 210 positioned at first andsecond angularly displaced positions represented respectively by solidand dashed lines. The alignment operation begins by moving robot arm 16to the first angularly displaced position at an angle Θ₁ from a lineextending between wafer center 252 and shoulder axis 260.

Robot arm 16 is extended in the r-axis direction and scanned in theZ-axis direction such that light transmission pathway 244 can sensewafer 12.

Robot arm 16 is then retracted in the r-axis direction to detect waferperipheral edge 18 and read a first extension distance r_(EXT1).

Robot arm 16 is moved to the second angularly displaced position at anangle Θ₂ from a line extending between wafer center 252 and shoulderaxis 260.

Robot arm 16 is extended in the r-axis direction and scanned in theZ-axis direction such that light transmission pathway 244 can sensewafer 12.

Robot arm 16 is then retracted in the r-axis direction to detect waferperipheral edge 18 and read a second extension distance r_(EXT2).

The above-described rotation and edge detection steps are repeated untilthe minimum distance r_(MIN) is determined between shoulder axis 260 andperiphery 18. This aligned position is shown in FIG. 13B.

Alternatively, after any two extension distances are known, the alignedposition can be converged upon by calculation and stored for future useby the controller. For example, for displacement angles Θ, correspondingextension distances r_(EXT) are known and, therefore, the particulardisplacement angle Θ at which extension distance r_(EXT) equals r_(MIN)can be calculated using the law of cosines. Of course, the alignedposition can be set manually and the position stored for future use bythe controller.

FIG. 13B shows robot arm 16 and end effector 210 in the alignedposition. From this position, the robot arm controller performs thefollowing movement operations to retrieve wafer 12.

The controller carries out operations to move centroid 262 of endeffector 210 along the imaginary line extending between shoulder axis260 and wafer center 252 until centroid 262 overlays wafer center 252.The required r-axis move distance is referred to as the offset distance,which is calculated as follows:

An r-axis distance between light transmission pathway 244 and distalrest pads 124 is a predetermined distance 264 established whenmanufacturing end effector 210.

Likewise, wafer 12 has a predetermined diameter 266.

Therefore, the offset distance is the sum of distance 264 and diameter266.

Assume that the controller has previously caused movement of endeffector 210 according to the above-described scanning and sensingoperations to locate and replace protruding wafers, determine wafer topand bottom surface z-axis locations and periphery 18 r-axis locations,determine wafer tilt angle, and to move to a position aligned with aselected wafer 12.

End effector 210 is moved to a z-axis location that clears the bottomsurface of the selected wafer 12 and the top surface of any adjacentwafer.

End effector 210 is moved the offset distance in the r-axis direction.

End effector 210 is moved a z-axis distance that causes proximal restpads 126 (FIG. 12) and distal rest pads 124 to contact wafer 12.

Active contact point 222 (FIG. 12) is activated to move wafer periphery18 into the included angles of distal rest pads 124, thereby edgegripping wafer 12.

End effector 210 retrieves wafer 12 with a movement in the r-axisdirection.

These above-described distance and alignment determinations areaccomplished without any of the teaching fixtures required by priorrobot arms and end effectors. If multiple end effectors 210 areemployed, the foregoing procedure can be repeated together withdetermining any Z-axis distance or elevation differences between them.

Light transmission pathway 244 may also be used in combination with thesupination capability of end effector 210 to determine whether acentroid 262 of end effector 210 is axially aligned with center 252 ofwafer 12B and shoulder axis 260 of robot arm 16. Ideally, centroid 262is coaxial with the center of a gripped wafer and lies on an imaginaryline extending between shoulder axis 260 and center 252 of wafer 12.However, manufacturing tolerances and the locations of features creatinglight transmission pathway 244 may cause a calculated position ofcentroid 262 to be offset from the supination axis of rotation.Determining whether centroid 262 is offset or coincident entailscarrying out the above-referenced alignment operations, rotating endeffector 210 through a supination angle 248 (FIG. 12) of 180 degrees andrepeating the alignment operations. If the centroid is offset, thecalculated alignments will be in a mirror image relationship on oppositesides of the supination axis of rotation. The correct location forcentroid 262 is determined by averaging the two calculated alignments.

Also, the above-described manufacturing tolerances and the locations offeatures creating light transmission pathway 244 may not be perfectlyaligned with tines 188 and 190, and there may be gripping errorsassociated with edge gripping end effectors (10, 110, 210). Such errorsmay cause up to about a 0.76 mm (0.030 inch) positioning error, which istypically side-to-side (about up to a 1-degree vector angle offset for a300 mm wafer) between the optically sensed and actual positions of wafercenter 252 relative to centroid 262. Location errors could also includeextension distance errors. For example, FIG. 13A shows typical angularoffset and extension distances.

It is desirable to return wafer 12 to its original location to optimizeprocess uniformity and to render pickup by another processing tooleasier and more accurate.

The following location correction method reduces or eliminates locationerrors.

Employ the method described with reference to FIGS. 13A and 13B forsensing and recording a cylindrical coordinate location {right arrowover (P)}_(OLD) of wafer 12.

Clamp wafer 12 in an edge gripping end effector and replace it in the{right arrow over (P)}_(OLD) location.

Again employ the method described with reference to FIGS. 13A and 13Bfor sensing and recording a cylindrical coordinate location {right arrowover (P)}_(NEW) of wafer 12.

Any location error is the geometric difference between the old and newlocations and represents the difference between the sensed and actuallocations of wafer 12. The location error is represented mathematicallyas:error={right arrow over (P)} _(NEW) −{right arrow over (P)} _(OLD)

Generating a correct location {right arrow over (P)}_(CORRECT) entailsadding the difference (error) back to {right arrow over (P)}_(OLD) tonegate the difference and then adding the difference (error) again tocorrect the error. This is represented mathematically as:{right arrow over (P)} _(CORRECT) ={right arrow over (P)}_(OLD)+2(error)

This mathematically simplifies as follows:{right arrow over (P)} _(CORRECT) ={right arrow over (P)}_(OLD)+2({right arrow over (P)} _(NEW) −{right arrow over (P)} _(OLD)){right arrow over (P)} _(CORRECT) ={right arrow over (P)} _(OLD)+2{rightarrow over (P)} _(NEW)−2{right arrow over (P)} _(OLD){right arrow over (P)} _(CORRECT)=2{right arrow over (P)} _(NEW) −{rightarrow over (P)} _(OLD)

Of course, the method can be carried out successively to iterativelyprocess the correct location information.

When the correct location is known, the wafer, or a suitable specimen,is retrieved from the new position with the end effector, and thecorrect location information employed to replace the specimen at thecorrect location.

The above-described embodiments are merely illustrative of theprinciples of the invention. Various modifications and changes may bemade thereto by those skilled in the art that will embody the principlesof the invention and fall within the spirit and scope thereof.

For example, skilled workers will understand that the pistons may beactuated by alternative power sources, such as, for example, by apulsing solenoid that slows the pistons as wafer 12 is secured. Electricsignals may be employed to drive and monitor the positioning of thepistons. The pistons may also be pneumatically operated and monitored,such as in applications where the end effectors are submerged in aliquid.

The end effectors may be forked or otherwise include a cutout or beshaped to avoid obstacles, such as a prealigner hub. The end effectorsmay by moved by devices other than robot arms, such as X-Y tables andother positioners have two or more degrees of freedom. Moreover, the endeffectors are usable for handling various types of specimens other thansemiconductor wafers, such as LCD display panels, compact diskettes, andcomputer memory discs, all of which may be stored in carriers other thanthe above-described cassettes.

The sensors preferably employ laser beams from light-emitting diodes anddiode lasers, but may also employ incandescent, infrared, and otherradiation sources.

The rest pad included angles are preferably acute angles, but thisinvention may include embodiments in which the specimens are held to theend effectors by gravitation force, in which instances the includedangles may be obtuse angles less than 180 degrees.

FIGS. 14A, 14B, 15, and 16 show a type of multiple link robot arm system308 to which end effector 210 is mountable. FIGS. 17A and 17B present inconjunction with pertinent mathematical expressions characterizing robotarm displacement an example of positioning robot arm mechanism 308 todemonstrate the manipulation of the linear and angular displacementvalues necessary to compute the parameters associated with the variouswafer sensing measurements described above. U.S. Pat. No. 5,765,444provides a detailed description of the construction and operation ofthis type of robot arm system.

FIGS. 14A and 14B are respective side elevation and plan views of atwo-arm, multiple link robot arm system 308 mounted on and through anaperture in the top surface of a support table 309. With reference toFIGS. 14A and 14B, two similar but independently controllable three-linkrobot arm mechanisms 310L and 310R are rotatably mounted at oppositeends of a torso link 311, which is mounted to the top surface of a basehousing 312 for rotation about a central or torso axis 313. Because theyare mirror images of each other, robot arm mechanisms 310L and 310R havecorresponding components identified by identical reference numeralsfollowed by the respective suffices “L” and “R”. Accordingly, thefollowing discussion is directed to the construction and operation ofonly robot arm mechanism 310R but is similarly applicable to robot armmechanism 310L.

Robot arm mechanism 310R comprises an upper arm 314R mounted to the topsurface of a cylindrical spacer 315R, which is positioned on theright-hand end of torso link 311 for rotation about a shoulder axis316R. Cylindrical spacer 315R provides room for the motors and certainother components of robot arm mechanism 310R, as will be describedbelow. Upper arm 314R has a distal end 318R to which a proximal end 320Rof a forearm 322R is mounted for rotation about an elbow axis 324R, andforearm 322R has a distal end 326R to which a proximal end 328R of endeffector or hand 210R is mounted for rotation about a wrist axis 332R.Hand 210R is equipped at its distal end 334R with a fluid pressureoutlet 336R that preferably applies vacuum pressure supplied to robotarm mechanism 310R at an inlet 338 to vacuum channel 164 to securelyhold semiconductor wafer 12, a compact disk, or other suitable specimen(not shown) in place on hand 210R. As will be described in detail later,each of upper arm 314R, forearm 322R, and hand 210R is capable ofcontinuous rotation about its respective shoulder axis 316R, elbow axis324R, and wrist axis 332R.

FIG. 15 shows the link components and associated mechanical linkage ofrobot arm mechanism 310R. With reference to FIG. 15, robot arm mechanism310R is positioned by first and second concentric motors 350R and 352Rthat operate in response to commands provided by a motor controller 354(FIGS. 17A and 17B). First motor 350R rotates forearm 322R about elbowaxis 324R, and second motor 352R rotates upper arm 314R about shoulderaxis 316R.

More specifically, first motor 350R rotates a forearm spindle 356R thatextends through an aperture in upper arm 314R and terminates in an upperarm pulley 358R. A post 360R extends upwardly at distal end 318R ofupper arm 314R through the center of a bearing 362R that is mounted to abottom surface 364R of forearm 322R at its proximal end 320R. Post 360Ralso extends through an aperture in forearm 322R and terminates in aforearm pulley 366R. An endless belt 368R connects upper arm pulley 358Rand the outer surface of bearing 362R to rotate forearm 322R about elbowaxis 324R in response to rotation of first motor 350R.

Second motor 352R rotates an upper arm spindle 380R that is mounted to abottom surface 382R of upper arm 314R to rotate upper arm 314R aboutshoulder axis 316R. Coordinated operation of first and second motors350R and 352R in conjunction with the mechanical linkage described belowcauses hand 210R to rotate about shoulder axis 316R. A post 384R extendsupwardly through the center of a bearing 386R that is mounted to abottom surface 388R of hand 210R. An endless belt 390R connects forearmpulley 366R to the outer surface of bearing 386R to rotate hand 210Rabout shoulder axis 316R in response to the coordinated rotationalmotions of motors 350R and 352R.

The mechanical linkage coupling upper arm 314R and forearm 322R forms anactive drive link and a passive drive link. The active drive linkincludes belt 368R connecting upper arm pulley 358R and the outersurface of bearing 362R and causes forearm 322R to rotate in response torotation of first motor 350R. The passive drive link includes belt 390Rconnecting forearm pulley 366R and the outer surface of bearing 386R andcauses hand 210R to rotate about wrist axis 332R in response to rotationof forearm 322R about elbow axis 324R. Rotation of hand 210R can also becaused by a complex interaction among the active and passive drive linksand the rotation of upper arm 314R in response to rotation of secondmotor 352R.

A third or torso motor 392 rotates a torso link spindle 394 that ismounted to a bottom surface of torso link 311, to which robot armmechanism 310R is rotatably mounted. A main ring supports a bearingassembly 398 around which spindle 394 rotates. Motor 392 is capable of360 degree continuous rotation about central axis 313 and therefore can,in cooperation with robot arm mechanism 310R, move hand 210R along anirregular path to any location within the reach of hand 210R.

Motor controller 54 (FIGS. 17A and 17B) controls motors 350R and 352R intwo preferred operational states to enable robot arm mechanism 310R toperform two principal motion sequences. The first motion sequencechanges the extension or radial position of hand 210R, and the secondmotion sequence changes the angular position of hand 210R relative toshoulder axis 316R. FIG. 16 is a useful diagram for showing the twomotion sequences.

With reference to FIGS. 15 and 16, in the first operational state, motorcontroller 354 causes first motor 350R to maintain the position offorearm spindle 356R and second motor 352R to rotate upper arm spindle380R. The non-rotation of first motor 350R maintains the position ofupper arm pulley 38R, and the rotation of upper arm spindle 38 OR bysecond motor 352R rotates upper arm 314R about shoulder axis 316R,thereby causing rotation of forearm 322R about elbow axis 324R andcounter-rotation of hand 210R about wrist axis 332R. Because the ratioof the diameters of upper arm pulley 358R and the outer surface ofbearing 362R are 4:2 and the ratio of the diameters of forearm pulley366R and the outer surface of bearing 386R is 1:2, the rotation of upperarm 314R in a direction specified by P2 shown in FIG. 16 will cause hand210R to move along a straight line path 400. (The diameters of forearmpulley 366R and the outer surface of bearing 386R are one-half of thediameters of, respectively, the outer surface of bearing 362R and upperarm pulley 358R to streamline the sizes and shapes of forearm 322R andhand 210R.)

Whenever upper arm 314R rotates in the clockwise direction specified byP2, hand 210R extends (i.e., increases radial distance from shoulderaxis 16R) along path 400. Whenever upper arm 314R rotates in thecounter-clockwise direction specified by P2, hand 210R retracts (i.e.,decreases radial distance from shoulder axis 316R) along path 400.Skilled persons will appreciate that robot arm mechanism 310 in a mirrorimage configuration of that shown in FIG. 16 would extend and retract inresponse to upper arm 314 rotation in directions opposite to thosedescribed. FIG. 14B shows that when robot arm mechanism 310R isextended, axes 313, 316R, 324R, and 332R are collinear.

In the second operational state, motor controller 352R causes firstmotor 350R to rotate forearm spindle 356R in the direction specified byP1 and second motor 352R to rotate upper arm spindle 380R in thedirection specified by P2. In the special case in which motors 350R and352R are synchronized to rotate in the same direction by the same amountof displacement, hand 210R is only angularly displaced about shoulderaxis 316R. This is so because the rotation of forearm 322R about elbowaxis 324R caused by the rotation of first motor 350R and the rotation ofhand 330R about wrist axis 332R caused by rotation of second motor 352Rand the operation of the passive drive link offset each other to produceno net rotation about elbow axis 324R and wrist axis 332R. Thus, hand210R is fixed radially at a point along path 400 and describes acircular path as only upper arm 314R rotates about shoulder axis 316R.By application of kinematic constraints to achieve a desired travel pathfor hand 210, motor controller 354 can operate first and second motors350R and 352R to move robot arm mechanism 310R along non-radial straightline paths, as will be further described below.

Skilled persons will appreciate that to operate robot arm mechanism310R, first and second motors 350R and 352R are coupled by eitherrotating both of them or grounding one while rotating the other one. Forexample, robot arm mechanism 310R can be operated such that forearm 322Rrotates about elbow axis 324R. Such motion would cause hand 210R todescribe a simple spiral path between shoulder axis 316R and the fullextension of hand 210R. This motion is accomplished by fixing theposition of shoulder 314R and operating motor 350R to move forearm 322R.

Motor controller 354 controls the operation of torso motor 392 andtherefore the rotation of torso link 311 in a direction specified by P3independently of the operational states of motors 350R and 352R.

The angular positions of motors 350R and 352R are tracked by separateglass scale encoders (not shown). Each of the encoders typicallyincludes an annular diffraction grating scale and a lightsource/detector subassembly (not shown). Such glass scale encoders areknown to skilled persons. The angular position of motor 392 is trackedby a glass scale the encoder of a type similar to the encoders formotors 350R and 352R.

FIG. 17A is a diagram that specifies a local coordinate axis frame whoseaxes are defined by the orientation of a semiconductor wafer cassette168 r and its location relative to shoulder axis 316R. With reference toFIG. 17A, the following description sets forth the mathematicalexpressions from which are derived the command signals controller 354uses to retrieve from cassette 168 r a wafer 170 r along a vectorperpendicular to the opening of cassette 168 r. (Skilled persons willappreciate that similar mathematical expressions can be used fordifferent drive ratios from the above-stated drive ratio on which thisexample is based.)

The following parameters are pertinent to the derivation of the path oftravel of hand 210:

-   -   Θ_(S)=angle of motor 352R    -   Θ_(E)=angle of motor 350R    -   r=distance between shoulder axis 316R and elbow axis 324R and        distance between elbow axis 324R and wrist axis 332R    -   β=angle between upper arm 314R and forearm 322R    -   p=length of hand 210R    -   E=2r=extension of robot arm    -   R_(i)=reach of robot arm (i.e., its radius measured from        shoulder axis 316R to the center 172 r of wafer 170 r positioned        on hand 210R).

Applying the law of cosines provides the following expressions forR_(i):R _(i) =p+{square root}{square root over ((r ² +r ²−2r ² cos β))}R _(i) =p+{square root}{square root over (2)}·r{square root}{square rootover ((1−cos β))}  (1)

For β=0, equation (1) provides that R_(i)=p, x=0, y=0, Θ_(S)=Θ_(SR), andΘ_(E)=Θ_(ER). The quantities Θ_(SR) and Θ_(ER) represent reference motorangles. The motor angles may be expressed as Θ_(S)=Θ_(SR)+ΔΘ_(SR),Θ_(ER)+ΔΘ_(ER). The angle β may be expressed as, β=2(ΔΘ_(SR)-ΔΘ_(ER))because of the construction of the mechanical linkages of robot armmechanism 310R. This equation relates the angle β to changes in themotor angles.

To retrieve wafer 170 r from cassette 168r along a straight line path,the displacement along the X-axis equals X₀, which is a constant. Thus,X(t)=X₀. The quantity X(t) can be expressed as a function of the lengthsof the X-axis components of its links:X(t)=r cos Θ₁ +r cos Θ₂ +p cos Θ_(p),  (2)in which

-   -   Θ₁=angle of upper arm 314R,    -   Θ₂=angle of forearm 322R, and    -   Θ_(p)=angle of hand 210R.

Because upper arm 314R and forearm 322R are of the same length (r), Θ₁tracks the angle Θ_(S) of motor 352R, and hand 210R moves in a straightline, the following expression holds:$\theta_{p} = {\theta_{1} + {\left( \frac{\pi - \beta}{2} \right).}}$

Thus, to compute X₀, one substitutes the foregoing identities for Θ₁,Θ₂, and Θ_(p) into equation (2) for X(t) and finds: $\begin{matrix}{X_{0} = {{r\left( {{\cos\quad\theta_{1}} + {\cos\quad\theta_{2}}} \right)} + {{p \cdot \cos}\quad\theta_{p}}}} & (3) \\{X_{0} = {{r\left( {{\cos\quad\theta_{1}} + {\cos\left( {\theta_{1} + \pi - \beta} \right)}} \right)} + {p \cdot {\cos\left( {\theta_{1} + \frac{\pi}{2} - \frac{\beta}{2}} \right)}}}} & (3) \\{X_{0} = {{r\left( {{\cos\quad\theta_{1}} - {\cos\left( {\theta_{1} - \beta} \right)}} \right)} - {p \cdot {{\sin\left( {\theta_{1} - \frac{\beta}{2}} \right)}.}}}} & (3)\end{matrix}$

Equation (3) expresses the constraint that sets out the relationshipbetween the angles Θ_(S) and Θ_(E) of motors 352R and 350R operating tomove equal angular distances to achieve straight line movement of hand210R.

Skilled persons can implement constraint equation (3) by means of aservomechanism controller in any one of a number of ways. For example,to achieve high speed operation to implement a given wafer move profile,one can compute from equation (3) command signal values and store themin a look-up table for real-time use. The precomputation process wouldentail the indexing of Θ_(S) in accordance with the wafer move profileand determining from equation (3) the corresponding Θ_(E) values,thereby configuring the displacement of Θ_(S) and Θ_(E) in amaster-slave relationship.

To achieve angular displacement of hand 210R about shoulder axis 316R,controller 354 causes motors 350R and 352R to rotate in the samedirection through the desired angular displacement of hand 330R to reachthe desired destination. The linear extension of hand 330R does notchange during this move. Skilled persons will appreciate thatcomplicated concurrent linear and angular displacement move profiles ofhand 330R could be accomplished by programming controller 354 to operatemotors 350R and 352R through different angular displacements.

FIG. 17A shows a second wafer cassette 168 _(L) positioned so that thecenter 172L of a stored wafer 170 _(L) is coincident to Y₀. The parallelarrangement of the openings of cassettes 168L and 168 _(r) demonstratesthat the above expressions can be used to retrieve wafers stored incassettes not positioned a radial distance from shoulder axis 316. Robotarm mechanism 310 is not restricted to radial placement but canaccommodate any combination of distances within its reach.

FIG. 17B is a simplified block diagram showing the primary components ofcontroller 354. With reference to FIG. 17B, controller 354 includes aprogram memory 474 that stores move sequence instructions for robot armmechanism 310R. A microprocessor 476 receives from program memory 474the move sequence instructions and interprets them to determine whetherthe first or second operational state is required or whether motion ofmotor 392 is required to position torso link 311. A system clock 478controls the operation of microprocessor 476. A look-up table (LUT) 480stores corresponding values for Θ_(S) (motor 352R) and Θ_(E) (motor350R) to accomplish the straight line motion of the first operationalstate and the angular displacements of Θ_(S) and Θ_(E) to accomplish theangular motion of the second operational state. Because the rotation oftorso link 311 is independent of the motions of the robot arm mechanismsmounted to it, the overall coordination of the angular displacement ofmotor 392 with the angular displacements of motors 350R and 352R iscarried out in the move sequence instructions, not in LUT 480. Thisresults in higher speed and more accurate straight line motion becausemultiple axis servomechanism following errors and drive accuracy errorsdo not affect the straight line path of hand 210R.

Microprocessor 476 provides Θ_(S) and Θ_(E) position signals to aservomechanism amplifier 482, which delivers Θ_(S) and Θ_(E) commandsignals to motors 352R and 350R, respectively. Microprocessor 476 alsoprovides position signals to servomechanism amplifier 476 to deliver acommand signal to torso motor 392. Servomechanism amplifier 482 receivesfrom the three glass scale encoders signals indicative of the angularpositions of the respective motors 350R, 352R, and 392.

Microprocessor 476 also provides control signals to a vacuum valvecontroller 484, which causes a vacuum valve (not shown) to provide froma vacuum source (not shown) an appropriate amount of vacuum pressure tooutlet 336 in response to the need to hold a wafer on or release a waferfrom hand 210R.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments withoutdeparting from the underlying principles of the invention. The scope ofthe present invention should, therefore, be determined only by thefollowing claims.

1. A method of correcting a robot arm positioning error, the robot armbeing operatively connected to an end effector that is operativelycoupled with a specimen, the method comprising: providing an endeffector having a body including a light source and a light receiverhaving spaced-apart respective source light path and receiver light pathopenings between which a light beam propagates along a straight linelight transmission pathway, which is employed to determine positions ofthe end effector relative to the specimen; determining a robot arm oldposition; recording old position information corresponding to the oldposition; retrieving the specimen from the old position with the endeffector; employing the old position information to replace the specimenat a new position that is ideally the same as the old position;determining the robot arm new position; recording new positioninformation corresponding to the robot arm new position; and generatinga correct position information by processing the new positioninformation and the old position information.
 2. The method of claim 1,further including determining a difference between the new positioninformation and the old position information to determine a positionerror information, and in which the generating a correct positioninformation includes processing the position error information and theold position information.
 3. The method of claim 1, further includingdetermining a difference between the new position information and theold position information to determine a position error information, andin which the generating a correct position information includes addingabout twice the position error information to the old positioninformation.
 4. The method of claim 1, in which the generating a correctposition information is carried out successively.
 5. The method of claim1, in which the processing includes vectorially combining the newposition information and the old position information.
 6. The method ofclaim 1, further including retrieving the specimen from the new positionwith the end effector, and employing the correct position information toreplace the specimen at a correct position.
 7. The method of claim 1, inwhich the specimen further includes a periphery, and determining therobot arm old position and new position entails finding a minimumdistance between a reference and the periphery of the specimen.
 8. Themethod of claim 7, in which determining the robot arm old and newpositions each includes: finding for a first robot arm position a firstmaximum robot arm distance between the reference and a firstcorresponding point on the periphery of the specimen at which the lighttransmission pathway of the light beam is not interrupted byintersection with the specimen; finding for a second robot arm positiona second maximum robot arm distance between the reference and a secondcorresponding point on the periphery of the specimen at which the lighttransmission pathway of the light beam is not interrupted byintersection with the specimen, the first and second robot arm positionsbeing different from each other; recording first and second positioninformation corresponding to the respective first and second maximumrobot arm distances; and determining from the first and second positioninformation a robot arm aligned position that represents the minimumdistance between the reference and the periphery of the specimen.
 9. Themethod of claim 8, in which the robot arm is positionable about ashoulder axis and along an r-axis path intersecting the shoulder axis,in which first and second robot arm positions constitute respectivefirst and second robot arm angular positions, and in which the findingof the first maximum robot arm distance includes: positioning the robotarm along the r-axis path until the specimen interrupts the lighttransmission pathway with the robot arm set to the first robot armangular position; and upon the interruption of the light transmissionpathway with the robot arm set at the first robot arm angular position,positioning the robot arm along the r-axis path to find the first pointon the periphery of the specimen and record the first informationcorresponding to the first maximum robot arm distance at the firstpoint.
 10. The method of claim 9, in which the shoulder axis constitutesthe reference location.